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Is it allowed and possible to hold at the intersection of two VOR radials using a single VOR receiver?
First of all, is it permitted? I am practicing VOR tracking on my simulator and the missed approach procedure (KLGB VOR 30) takes you out to an intersection off two VOR radials. Since the plate doesn't specify "DME required" I figured yes.
Assuming yes, are there any tricks/protips to doing this successfully?

Is there a reference to something that specifically disallows cross-tuning for a holding fix?

I know it was allowed at one time. I remember a documentary of a crash that occurred in the 70s or 80s. The narrator mentioned that the plane only had one working VOR receiver, so the copilot was distracted by the need to constantly switch frequencies to maintain their hold, and that this was legal. I don't know if they've changed the rules since then or not, and I can't even find the documentary anywhere.

@HiddenWindshield That's my recollection as well (legal at one time), and why I upvoted the question. I vaguely remember a statement about you can crosstune up until the FAF, but not past it. Not something I'd relish having to do in the weather, but legal at the time. Hopefully we get a definitive answer to this for today's rules.

As I understand it you are asking two questions:

is it permitted to hold at an intersection of two VOR radials with nothing more than a single VOR receiver? and
what are some tips and tricks for doing it?

Answer to question one:
As others have mentioned, it was permitted at one time (and I believe it is still permitted). I'm old enough to have received training on how to do it at a time where it was much more common to fly in GA aircraft with no DME or GPS and sometimes only a single VOR receiver. I spent over an hour researching this question for you in the AIM, Airplane Flying Handbook, Instrument Flying Handbook, and Instrument Procedures Handbook, and online generally, and was not able to find anything saying it was permitted or prohibited. I don't see any problem with it other than it adds a bit of workload to the pilot, but it can be done safely and relatively easily. If I find an definitive answer I'll update my question.
Answer to question two:
Here are some tips and tricks for flying an intersection hold with nothing but a single VOR receiver. This explanation assumes you know how to hold, including the FAA recommended hold entries.
There are two radials, hence the intersection. I'm going to refer to them here as the main radial and the cross radial. (It is possible to approach the fix on the cross radial then switch to the main radial for the hold after executing the hold entry, however, for purposes of this explanation, I'm going to assume that you are approaching the fix on the main radial (i.e. the radial on which the inbound holding course is located).
In the figure below the aircraft is on the main radial and is approaching the cross radial from the West. However, this aircraft could also be approaching from the opposite side of the fix from the East.

So, while you are on the main radial flying towards the fix, you want to spend most of your time monitoring the main radial because it is your primary course navigation, however, you also want to initially tune the cross radial well before reaching it in order to verify, and take a mental note of which side the CDI is deflected. This can be confusing to some pilots because the CDI will be either on the right or the left depending on where the station of the cross radial is located in relation the main radial. (When you fly a G1000 or HSI, the CDI rotates such that it is easy to tell when you are approaching the cross radial or have already passed it, but this is not the case in stand-alone old-school VORs). Whether the CDI of the cross radial is on the left or right also depends on if the pilot tuned in the radial or the reciprocal. For simplicity in this example, lets assume you tune in the cross radial well before reaching the intersection and the CDI is fully deflected to the left.
You then re-tune the main radial (hopefully your NAV radio has a flip-flop button), and center the CDI. You want to be as precise as possible and note the wind correction angle required and bug that heading! It's important to be precise, because you will be flying the bugged heading when you switch to the cross radial.
As you fly towards the holding fix on the main radial you will periodically switch back and forth to the cross radial until you detect the CDI is alive. At this point, you want to watch the CDI to get an idea of how fast it's moving.  I like to take a note of the deflection of the CDI, lets say it is 8 degrees to the left (still haven't reached it), then take a note of the clock to determine how long it takes to fly 1 degree. Let's say that it takes 30 seconds to go from 8 degrees to 7 degrees, if so, then you know you have approximately 3.5 minutes before reaching the holding fix (7 degrees x 30 sec/deg = 210 sec or 3.5 min). You can then switch back to the main radial and continue navigating on it, until about 1 minute before you reach the intersection, at which point you would tune in the cross radial and fly your bugged heading until the cross radial is centered. At which point you would peform the appropriate hold entry.
At the moment you reach the holding fix, you will be flying the bugged heading and the cross radial will be tuned into your VOR. If you are on the main radial on the inbound heading, then it will be a direct entry and you just turn (right turn in the above figure) and fly outbound. On the outbound leg, tune the VOR to the main radial to the inbound course so that you can intercept it when you turn inbound. If you are approaching the holding fix from the opposite direction (from the East in the above figure) such that you are flying the outbound heading of the hold, then you will need to do a teardrop or parallel entry. In this case, I like to do the parallel entry because it requires less math and turning. Since you are already flying the outbound heading and are parallel to the inbound course you don't have to do anything when you reach the holding fix from this direction other than start the timer or take a note of the time and fly out for 1 minute. During this one minute of flying outbound on the parallel entry, you will tune in the main radial to the inbound course, so that when you make your greater than 180 degree turn you can intercept the inbound course.
Whether you do the direct entry or parallel entry, once you are established on the inbound course and tuned into the main radial, you will need to bug in that heading, and promptly tune the VOR to the cross radial, because you've only got a minute or so on the inbound leg to intercept the inbound course, determine the WCA, bug it, then tune the cross radial to identify reaching the holding fix.
Then repeat until you're ready to leave the hold.
This was a long explanation, but it's really not as bad as it sounds. When you are practicing this on the sim, you can try flying 2 minute legs, which gives you more time to intercept inbound, then tune the cross radial. As you get more efficient, you can shorten the legs until you get to 1 min. BTW, in actual IFR, you can ask ATC for a non-standard leg length. I imagine any airplane in this day an age with no DME or GPS that only has a single VOR is probably a very very slow airplane, and ATC can probably accomodate 2 or 3+ minute legs, just ask.

Yes, you can hold at an intersection with only the radial from one VOR  identifying the intersection as long as you have either DME or GPS. And yes, you can hold at an intersection without DME or GPS if you have the intersection identified by the radials of two VORs. You can even hold with nothing but a GPS alone.
If you want to hold at a non-published hold position of your choosing, you could do so at a specific distance and direction from any identifiable point along a specific bearing from your hold fix. You can substitute the distance with the bearing from a second identifiable point. This includes NDBs, airports, Visual Reference Points, GPS waypoints, etc.
I always find it easiest to track inbound to the fix using either a radial, bearing, or GPS direct-to to the fix. When using VOR and DME, it is easiest to intercept the identifying radial way prior to reaching the appropriate distance. If you are using two VORs to identify the hold fix, intercept one of the identifying radials using one of your OBSs first. Use your other OBS to identify your second radial. Which OBS you choose depends on the radial or bearing of the fix along which you will be holding. That should be your primary OBS.
One of the holds given to me by the DPE on my instrument checkride was an unpublished hold at a self-declared, unofficial waypoint/fix. Since, I was on an IFR flight plan, I had to request the hold from ATC with the following format.

Skyhawkxxxxx, requesting a hold xx direction on a ### bearing, ## nautical miles and XX direction from intersection xxxxx, along the ### radial of xxxVOR, 1 minute legs, standard turn, at #### feet.

Admittedly, this was a bit of an over the top scenario given by the DPE. He was a legacy carrier captain who believed in making sure he tested beyond the capabilities of the magenta-line generation. It was more of a test of how I could think through, draw out , and communicate the hold while maintaining directional control under the hood rather than how to fly it. The bearing from the fix put me right in line with the IAF of my first IAP that had a hold-in-leau of procedure turn anyway.

Along with the info about all the things one can do in scenarios that are NOT asked about, does this answer actually give a "yes" or "no" answer to the stated "single receiver" (presumably without any other equipment like ADF, DME, or GPS) question?

@RalphJ - Actually, yes. The first paragraph answers the question of “Is it allowed and possible to hold at the intersection of two VOR radials using a single VOR receiver?“, as edited by Pondlife, with a Yes. It also answers the question “Is it possible to hold at an intersection with one VOR?“ as originally posted with a Yes. The third paragraph answers the question “Assuming yes, are there any tricks/protips to doing this successfully?“

@RalphJ - If the OP’s question is specifically whether you are allowed to identify the holding fix using only one operational VOR receiver and one operational OBS display, the answer is still yes. Just as long as you have either a way to know distance, or there is a crossing VOR radial from another VOR transmitter. Flip-flopping VOR frequencies and radials is not the preferred method of doing this (especially IMC). But, there are still a lot of planes flying with only one NavCom radio (C150/2, many LSAs, etc). Just establish yourself on the hold’s inbound radial first before cross-referencing.

I fully understand HOW one would do it; the interesting question is, is such an operation permitted with the single receiver + no GPS, DME, or other navigation receivers. And, ideally, a link to something authoritative stating that it is (or isn't) permitted to identify a holding fix in this manner. Not just "yes you can" but "yes you may, per {this reference}".

@RalphJ - Both the AIM 5-4-5 and the Instrument Flying Handbook both contain similar verbiage: “More than one navigation system separated by a slant indicates that more than one type of equipment must be used to execute the
final approach (for example, VOR/DME RWY 31). More than one navigation system separated by the word“or”indicates either type of equipment can be used to execute the final approach (for example, VOR or GPS RWY 15).” I am unable to find an IAP labeled VOR/VOR at this time.

@RalphJ - 14 CFR 91.171 (c) States: “If dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, the person checking the equipment may check one system against the other in place of the check procedures specified in paragraph (b) of this section. Both systems shall be tuned to the same VOR ground facility and note the indicated bearings to that station. The maximum permissible variation between the two indicated bearings is 4 degrees.“ If in the above info implies that having 1 OBS is neither prohibited nor unexpected. There may be something in 121

Consider this, most Cessna 150 & 152 only have one radio. The same applies to many LSAs. Those aircraft as well as Aspen, Dynon, and G500/G1000 aircraft only have 1 CDI or 1 visible at a time (not counting the bearing functions). Even without a DME or GPS, there is no prohibition against shooting a VOR approach not designated as needing additional equipment. If it is not prohibited, then permission is implied. If there is a regulation that you can site, or a subsequent advisory circular, I will stand corrected and change my answer. So far, I have found none.


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Can I include my point of departure in the three landings for my commercial solo cross-country?
I flew a cross-country with one stop on the way down: A -> B -> C
From A to C is over 250nm in a straight line and over 300nm total.
The next day I flew back: C -> A
That covers the 3 landings at intermediate (B), destination (C) and back at departure (A). But does the landing back at the departure location count as one of the three?

I assume you're asking about FAA regulations so I added that tag. If I got it wrong please edit your question to add the correct country; we always need to know the country for questions about regulations.

Possible duplicate -- or at least highly related-- https://aviation.stackexchange.com/questions/55825/what-defines-the-end-of-a-cross-country-flight-for-ppl-aeronautical-experience

It depends on how you classify the flight. More importantly, it depends on how the DPE classifies the flight. Most people consider any flight with a stop longer than a fuel break as the end of the flight at the fuel stop, and the beginning of a new flight at the subsequent start up. Some sticklers would consider anytime you shut down the engine on the ground, the end of that flight.
You can log it any way you want. I doubt any DPE will count a flight with an overnight stopover between legs as one flight.
Here is an alternative. Takeoff at Airport A. Fly a closed traffic pattern to a full-stop, taxi-back landing. Fly to Airport B where you perform a full-stop, taxi-back landing. Fly to Airport C where you perform a full-stop landing. As long as the straight line distance between A and C is more than 250 Nautical Miles, your total distance is over 300 Nautical Miles, and you are either solo or performing the duties of PIC with an instructor, you should be good for the FAA Title 14 CFR Part 61.129(a)(4)(i) requirements for a commercial pilot.
Note that the regs don’t stipulate a full stop, taxi-backs. If you are flying solo, it gives you time to set up for the next leg. Although it is hard to believe that there is not another suitable airport in 300 Nautical Miles. In what area are you flying?

I consider it part of the same flight even if I don't get back to the plane till two days later.  Maybe someone else flies it in between; doesn't matter.  Maybe I make a few circuits around the airport the next day and then make another cross-country flight the following day-- as far as I'm concerned all those hours including the hours in the local circuits get logged as part of the same cross-country flight.  The FAA has provided no clear definition as to when a cross-country flight should be considered to have ended.

Basically, I've been on one long cross-country flight for the last 10 yrs or so.

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How do class E/G airfields charge for stops without refuel?
Do many Class E/G airfields charge for short stops without gas purchase? Sorry for the stupid question...recent PPL here and my training has mostly been T&G rather than full stops to the FBO. Planning a XC to KSHD and wanted to stop in for a bit to check out the facility.

You can (and probably should) always call and ask.   Their policies may change from time to time, and on a given day, they could be extra busy (and not have space available) or extra slow, and welcome any traffic.

E and G are generally free, in my experience. They just want the traffic.  Some have overnight fees (usually on the honor system), most don't.  The cost to enforce exceeds the revenue.

I have found this really varies by airport and even by FBO at a given airport. The only way to know is either to check their website or call in advance and find out. After a quick look I don't see any prices listed on their site.
Often times the price is related to the services they provide by default. If you land and there is a marshaler waiting for you, a golf cart ride from your spot to the FBO building and loads of free local snacks: you're likely to be paying. If you land and the airport manager lives next door but his cell phone number is written on a sign that says "call for assistance", the fuel is self serve, and the gate code is the CTAF frequency: you're likely not paying for parking.
The one thing that's worth noting is if there is a local (usually nominal) landing fee charged by the municipality the airport resides in.
AOPA's airport information site has some fee info on it. For example KLOM has some fairly high fees for a class E un-towered field. While I can confirm from experience that the $5 ramp/parking fee at KDDH is waved for buying fuel.

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How far along was JFK Jr. in his instrument training?
I read up a little more about the infamous and tragic incident. JFK Jr. had over 300 hours logged, and about 36 in the Saratoga, half of which was solo. It's not like he was a newly minted VFR pilot and supposedly his CFI thought he was an "excellent" pilot.
However, there was very little information on how far along he was in his instrument training. Did he just get the minimum requisite time under the hood? Did he begin pursuing IR training?

The kind of IMC conditions he encountered could be the worst trap possible.  He encountered "greyout" in haze where you lose your horizon reference but you're not actually in cloud so you don't have the sense that you're in real IMC, and you may not make the mental gear change required to go from looking outside to looking inside, at least not right away, and this can lead to the initial meandering off course that starts to get the vertigo effects working, building up slowly to the world tumbling all around you.

A little over 300 hours in almost 12 years is not a lot of hours. The principle of disuse will set in. And, very little, flight time was done in actual IMC. If this was his first time in actual IMC as PIC, being single pilot is a recipe for disaster even if he had completed his IR. Which he hadn’t. Then, there has to be a reason why he had such inconsistency in who he received training from. What was consistent is his CFIs opinions on his ability to handle task saturation.

@DeanF. He had about 260 hours in less than 2 years, which is a lot for a PPL.

From the NTSB accident report:

Pilot Training
On October 4, 1982, the pilot started receiving flight instruction. Over the next 6 years, he flew with six different CFIs. During this period, the pilot logged 47 hours, consisting of 46 hours of dual instruction and 1 hour without a CFI on board. The pilot made no entries in his logbook from September 1988 to December 1997.
In December 1997, the pilot enrolled in a training program at Flight Safety International (FSI), Vero Beach, Florida, to obtain his private pilot certificate. Between December 1997 and April 1998, the pilot flew about 53 hours, of which 43 were flown with a CFI on board. The CFI who prepared the pilot for his private pilot checkride stated that the pilot had "very good" flying skills for his level of experience.
On April 22, 1998, the pilot passed his private pilot flight test. The designated pilot examiner who administered the checkride stated that as part of the flight test, the pilot conducted two unusual attitude recoveries. The pilot examiner stated that in both cases, the pilot recovered the airplane while wearing a hood and referencing the airplane's flight instruments. After receiving his private pilot certificate, the pilot flew solo in his Cessna 182 and received instruction in it by CFIs local to New Jersey. He also received instruction at Million Air, a flight school in New Jersey, and flew their airplanes. During calendar year 1998, the pilot flew approximately 179 hours, including about 65 hours without a CFI on board. On March 12, 1999, the pilot completed the FAA's written airplane instrument examination and received a score of 78 percent.
On April 5, 1999, the pilot returned to FSI to begin an airplane instrument rating course. During the instrument training, the pilot satisfactorily completed the first 12 of 25 lesson plans. The pilot's primary CFI during the instrument training stated that the pilot's progression was normal and that he grasped all of the basic skills needed to complete the course; however, the CFI did recall the pilot having difficulty completing lesson 11, which was designed to develop a student's knowledge of very high frequency omnidirectional radio range (VOR) and nondirectional beacon operations while working with ATC. It took the pilot four attempts to complete lesson 11 satisfactorily. After two of the attempts, the pilot took a 1-week break. After this break, the pilot repeated lesson 11 two more times. The CFI stated that the pilot's basic instrument flying skills and simulator work were excellent. However, the CFI stated that the pilot had trouble managing multiple tasks while flying, which he felt was normal for the pilot's level of experience.
The pilot attended this training primarily on weekends. During this training, the pilot accumulated 13.3 hours of flight time with a CFI on board. In addition, the pilot logged 16.9 hours of simulator time. The pilot departed from FSI for the last time on April 24, 1999.
The pilot continued to receive flight instruction from CFIs in New Jersey in his newly purchased Piper Saratoga, the accident airplane. One CFI flew with the pilot on three occasions. One of the flights was on June 25, 1999, from CDW to MVY. The CFI stated that the departure, en route, and descent portions of the flight were executed in VMC, but an instrument approach was required into MVY because of a 300-foot overcast ceiling. The CFI requested an instrument flight rules (IFR) clearance and demonstrated a coupled instrument landing system (ILS) approach to runway 24. The CFI stated that the pilot performed the landing, but he had to assist with the rudders because of the pilot's injured ankle. (For additional information about the pilot's ankle injury, see Section, "Medical and Pathological Information.") The CFI stated that the pilot's aeronautical abilities and his ability to handle multiple tasks while flying were average for his level of experience.
A second CFI flew with the pilot between May 1998 and July 1999. This CFI accumulated 39 hours of flight time with the pilot, including 21 hours of night flight and 0.9 hour flown in instrument meteorological conditions (IMC). The pilot used this CFI for instruction on cross-country flights and as a safety pilot. On July 1, 1999, the CFI flew with the pilot in the accident airplane to MVY. The flight was conducted at night, and IMC prevailed at the airport. The CFI stated that, during the flight, the pilot used and seemed competent with the autopilot. The instructor added that during the flight the pilot was wearing a nonplaster cast on his leg, which required the CFI to taxi the airplane and assist the pilot with the landing.
The CFI stated that the pilot had the ability to fly the airplane without a visible horizon but may have had difficulty performing additional tasks under such conditions. He also stated that the pilot was not ready for an instrument evaluation as of July 1, 1999, and needed additional training. The CFI was not aware of the pilot conducting any flight in the accident airplane without an instructor on board. He also stated that he would not have felt comfortable with the accident pilot conducting night flight operations on a route similar to the one flown on, and in weather conditions similar to those that existed on, the night of the accident. The CFI further stated that he had talked to the pilot on the day of the accident and offered to fly with him on the accident flight. He stated that the accident pilot replied that "he wanted to do it alone."
A third CFI flew with the pilot between May 1998 and July 1999. This CFI accumulated 57 hours of flight time with the pilot, including 17 hours of night flight and 8 hours flown in IMC. The pilot also used this instructor for instruction on cross-country flights and as a safety pilot. This CFI had conducted a "complex airplane" evaluation on the pilot and signed him off in the accident airplane in May 1999. According to the CFI, on one or two occasions, the airplane's autopilot turned to a heading other than the one selected, which required the autopilot to be disengaged and then reengaged. He stated that it seemed as if the autopilot had independently changed from one navigation mode to another. He also stated that he did not feel that the problem was significant because it only happened once or twice.
The CFI had made six or seven flights to MVY with the pilot in the accident airplane. The CFI stated that most of the flights were conducted at night and that, during the flights, the pilot did not have any trouble flying the airplane. The instructor stated that the pilot was methodical about his flight planning and that he was very cautious about his aviation decision-making. The CFI stated that the pilot had the capability to conduct a night flight to MVY as long as a visible horizon existed.

So he had enrolled in a school to earn an IR training, and he had completed 12 of the 25 lesson plans, meaning he was about half way through training. His total experience, as of the log book that was available, was 310 hours, 55 hours at night. He had 36 hours in the accident aircraft (9.4 at night). About 47 of his hours took place between 1982 and 1988, he did not fly between 1988 and 1997. The NTSB report does not contain his total IFR hours.

Thanks very much for this. Two things of interest: JFK Jr. knew how to use the AP yet decided to not even engage the wing leveler, which would've easily provided some offload of multitasking. He also opted to have his wife sit in back with her sister, instead of having her as right seat helper (nav/com radios, etc). Very odd.

@saigafreak Not necessarily.  It’s possible that he simply wanted to hand fly the airplane that evening.  I don’t know if having both Caroline and Lauren seated in the back was JFK Jr’s plan but it’s reasonable to believe they chose that arrangement as it offered them more legroom (you could stretch out in the PA-32’s club seating better that sitting up front)

@saigafreak Depends on the AP, the KP150 (in JFK Jr's aircraft) did not have a "straight and level" button, so there was no "wing leveler". Also putting a non-pilot trained passenger in charge of radios or maps can increase your workload instead of reduce it. If you want to have a non-pilot passenger help out with radio's, the best time to teach them is on the ground, or in a pinch-hitter course offered by the AOPA.

As Ron Beyer’s answer stated, JFK Jr. was roughly halfway through the practical portion of his instrument training.  He had completed the knowledge test but was not ready to do the checkride.  What happened to him was really getting into an extremely insidious situation which, based on his flying experience, it’s reasonable to believe he could not have foreseen.  The flight was supposed to be conducted during day VFR conditions but the departure was delayed for at least two hours as John, Caroline, and Lauren were stuck in terrible traffic in NYC that evening and could not reach Essex Co. airport at the original time they planned.  Add to this hazy, marginal weather, external pressures to be at their destination that evening for Rory’s wedding rehearsal in Hyannis, and John not fully recovered from a paraglider accident 5 weeks earlier which broke his ankle, he got lured into something he was totally unprepared for.
The accident is a cautionary tale about flying over dark water at night.  And even if John had been a competent instrument rated pilot, he may not have been ready to handle.  I’ve always described dark night over water as an eerie experience.  You go straight into a black hole with nothing outside the cockpit to tell you which way is up.  Combine that with a marine layer forming over the area near Martha’s Vineyard and the additional cockpit tasks of descending and preparation for landing, it’s easy to understand how he became spatially disoriented.


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What is the role of a CFI (not CFII) as safety pilot for hood time?
I am an instrument student at a flight school with a "primary" CFII whom I'm logging dual instruction time. Things are going pretty well, making progress but the instructor has been unavailable a bit more lately. Now, I also belong to a flying club next door at the same airport where the planes are much cheaper but double-I instructor availability is not as prevalent. My checkout instructor at this club has been giving me hood time. The rental cost of the plane at the club plus non double-I instructor time is actually more cost effective, and scheduling has been more of a breeze than getting dual time with the flight school's instructor. My flight school instructor looked at my logbook and did a double-take on a recent entry with the checkout instructor. I'm guessing he's not happy because I'm not doing business with his flight school 100% and renting their higher rate airplanes with a safety pilot (i.e. his other IR students). My double I is encouraging me to do some safety pilot time with other IR students, which obviously isn't a bad idea either for some folks. From my standpoint, I didn't want to do as much safety pilot time at the school since we trade off (i.e. I need 2x the amount of free time), and I figure I get the value-added of CFI coaching at the club. BTW this checkout CFI is about 1-2 months away from his double I and is actually signed off on giving me FTD dual instruction. Hopefully this makes sense so far...
I guess the questions that arise are: would this be a red flag for you as an instrument student? Is it really a big issue if a non-II CFI provides mentoring along with being a safety pilot for the simulated hours that is NOT dual instructor allocated (i.e. the other 25 hours)? Is the CFI in a legal bind somehow because they should "know better" if they let me as PIC do something wrong?

I added the FAA regulations tag because I believe you're asking about the US? Please always tell us which country you're asking about for questions around regulations, standard practices etc.

Indeed. US only and under VMC

By chance, is the CFI either an Instrument Ground Instructor or and Advanced Ground Instructor?

The bottom line is that it is up to you. It would be very beneficial to get as much simulated IFR time as possible. It does not matter if it is with your school CFII, a school student, the club CFI, a club student, or an unaffiliated and current pilot friend. Your CFII should not have a problem with that. Instruction (and the cost of it) is about you, not him. Although, they may have a legitimate concern about the quality of training you are receiving outside of the school. And, be wary of training in aircraft that are too different from one another, even in the same make and model.
The benefits of doing IFR time with your school CFII are:

It counts towards dual instruction time.
Your instruction stays consistent in structure.
The CFII will be able to give you the gouge (predisposition of questioning and testing) of the DPE.
The CFII is already familiar with your deficiencies.

The benefits of doing IFR time with the club CFI:

It counts as PIC and IFR time even if it does not count as dual instruction.
Costs less than the school.
You get a different perspective, tools, tips and tricks from an instructor qualified, instrument rated pilot.
A new/different set of eyes may uncover different deficiencies.
Typically, club instructors are going to stick around and have more longevity than school instructors. Although, this may not always be the case. It will give you an opportunity to build camaraderie and community for the future. After all, who is going to give you your Flight Reviews and Instrument Proficiency Checks.

The benefits of doing IFR time with another student:

Another student may have picked up something different but useful in training than you did.
Collaborative learning.
Building community and networking for the future.
Cuts out the cost of an instructor.
If it is a school student, their training will be consistent with yours (supposedly).
If it is a club student, they may have been trained completely differently. You may be able to glean useful tidbits from that different training.
You get to double your total flight/PIC time when trading off safety pilot-flying pilot responsibilities.
Flying! ‘Nough said.

The benefits of doing IFR time with a current and proficient, unaffiliated pilot friend:

Again, Flying! Flying is winning.
Totally different perspective. Remember, a good pilot is always learning, even from other pilots.
Camaraderie, community, networking, friendships, and that $100 burger/breakfast/BBQ.
You can fly any aircraft that you want. Although, it may not count as total and/or PIC time if you are not certificated for that category, class, and type rating if applicable unless the friend is also a CFI.
And of course, money. You can fly something cheap. Ty to find someone who owns their own aircraft.

Lastly, never overlook or underestimate the value of just sitting in the backseat with your headphones on and your mouth shut during another student’s lesson. You may pick up a lot of valuable information when you don’t have the added burden of flying the aircraft. Even if you can’t actually log the time.
As far as the “legal bind” that you mentioned. Any and all pilots acting as PIC will be in a legal bind if something goes wrong. After all, they are PIC for the purposes of safety of flight. That is the responsibility of their certificate. Any PIC who currently or in the future earns a living from their commercial license of any type will have more at stake than a recreational pilot.

Flying under the hood with a safety pilot shouldn't be considered as receiving training.  Ideally, you should be acting as though the safety pilot isn't even there.

@jamesqf - Flying with foggles with a safety pilot is not considered as receiving training. I never said nor implied it was. Receiving training from a certificated instructor while flying with foggles is considered receiving training. Although, anytime you are flying, regardless of who with or even if you are flying with no one, you should be learning something. Even if it is not considered training. Even as PIC. The learning and knowledge you get from the sheer experience is invaluable. And, the hood time can still be logged as simulates instrument time without the instructor.


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Aspect Ratio conceptual estimate
so I know that AR=b^2/S.
But let’s say that I don’t know b or S, how can I estimate the optimal AR if I know only this:
How many passengers are going to be onboard, cruise velocity (subsonic), altitude, range and a very rough configuration.
Endurance: around 500km worth
Cruise: 180knots
Altitude: 2km
Payload: 220lb
Is there something that is used just to help estimate please, I read a book and it said ‘initial estimate made from historical data’?

Do you mean how would you estimate the optimal aspect ratio in such a case?  If so, please clarify.  The question as posted is not answerable.

PS even just to suggest the optimal aspect ratio, it appears we might also need to know things like intended cruise altitude, intended range, etc- basically it appears you are asking us to design the whole airplane for you based only on the number of passengers, which appears to be way too broad a question.

Hi, sorry I haven’t included numbers on purpose. I just wanted an indication of how to do it please so I could try myself first. Then come back if I need help

Ok, I just suspect we may need more info.  Perhaps not, perhaps someone will post an answer showing that what you have provided is all we need to know.  But again that would be for some sort of optimal aspect ratio.  (See first comment.). Well, we'll see how this goes...

Hi, about the optimal AR, how is that obtained please?

I see you've made some substantial edits - it still seems a question worthy of an entire textbook- but maybe one of our talented contributors will be able to help you out-

Ps I made an edit as it seemed pretty clear that's what you meant, but if not please roll back or let me know so I can roll back.

@PeterCashell -  Ok.  So there is a principal clue that you can use to estimate this.  Primarily, the approach to take is to back out the wing configuration from the wing loading.

Hi, sorry I don’t get what you mean by back out please?

"Estimate using historical data" means pick some existing aircraft that are similar to what you are designing and see what they did. This will give you a ballpark number for aspect ratio (and other useful numbers) which should work. You then adjust for factors you want to prioritize.

What you do in this case is what every conceptual designer does: Use parametric data. This is a compilation of data from similar designs which yields numerical value equations (= the units on both sides don't match). Typical collections are the series of books published by Jan Roskam. But you could as well compile your own, using data from Jane's or Wikipedia.
As an example, here are the equations for wing mass $\mathsf{m_W}$, valid for small, propeller-driven airplanes, from Roskam's book:
$$\mathsf{Strutted:}\;\mathsf{m_W}\mathsf{=0,015\cdot\left(1,2-\frac{m_{Fuel}}{m}\right)\cdot\left(1,5\cdot n_{z_{max}}\right)^{0,611}\cdot S^{1,018}\cdot AR^{2,473}}$$
$$\mathsf{Cantilever:\;m_W\:}\mathsf{\mathsf{=0,068\cdot m^{0,397}\cdot\left(1,2-\frac{m_{Fuel}}{m}\right)\cdot\left(1,5\cdot n_{z_{max}}\right)^{0,397}\cdot S^{0,36}\cdot AR^{1,712}}}$$
Interestingly, the equation for the strutted wing only depends on fuel fraction $\mathsf{\frac{m_{Fuel}}{m}}$, load factor $\mathsf{n_{z_{max}}}$, wing area $\mathsf{S}$ and aspect ratio $\mathsf{AR}$, not the speed or range of the airplane. This shows that all data used for this equation came from designs with similar range and speed.
The equation for the cantilever wing already includes the take-off mass $\mathsf{m}$ by itself but is similarly from a small range of designs.
Since the units of both sides don't match, you need to be double sure to use the correct dimensions in this type of equations. Here, they are valid vor proper SI units: Kilogramm and Meter.
Starting with the number of passengers and a rough idea of the range, you can calculate the payload (100 kg/pax for short range or 161 kg per m³ of cargo volume) and from the payload fraction of similar airplanes you get a first estimate for take-off mass (MTOW).
Since you seem to have a fast, unpressurized single-seater in mind, we could as well calculate the mass of all parts. Fuselage mass $\mathsf{m_F}$ for small airplanes, according to Roskam, is
$$\mathsf{m_F=0,08378\cdot\left(n_{z_{max}}\cdot m\right)^{0,315}\cdot l_F^{0,943}\cdot\left(h_F+w_F\right)^{1,1}\cdot v_{D}^{0,372}}$$
Here we find length, height and width ($\mathsf{l_F, h_F, w_F}$) of the fuselage and the dive speed $\mathsf{v_D}$, so another unit to clarify is Seconds. $\mathsf{v_D}$ has to be in $\mathsf{\frac{m}{s}}$.
To complete the mass calculation you need to add empennage mass, systems (glass cockpit or only a few steam gauges? Retractable or fixed gear?) and engine. The power needed can be derived from the L/D of the plane at this speed, propeller efficiency and airplane mass. Let's assume an L/D of 8 and a propeller efficiency of 0.7 (follow the links to see how optimistic those estimates are!) and you have the installed power $\mathsf{P}$ needed. From that it is only a small step to engine mass $\mathsf{m_E}$:
$$\mathsf{m_E=0,68\cdot P\cdot\frac{2400}{n_{max}}}$$
with $\mathsf{n_{max}}$ the maximum RPM of the chosen engine. The range and fuel consumption of the engine tell you how much fuel to add. Once you know this, your parametric tank mass is
$$\mathsf{m_{Tank}=0,614\cdot m_{Fuel}^{0,667}}$$
Once you have an idea of the take-off mass, you look up typical span loadings (kg of MTOW per m of wingspan) and arrive at a first estimate for your wingspan. Next, you make some heroic assumptions about the wing's sweep (should be zero), airfoil and flap system to be used and, together with the maximum lift coefficient those provide, you calculate a wing area that both allows for a desired minimum speed and cruising altitude. With that information, you can go back to mass estimates and get a better idea of your wing weight and the thrust needed, allowing you to narrow down the estimate for engine weight. Given your relatively high cruising speed of 92.6 m/s, you should aim for a small wing with powerful flaps.
That helps you to update the previous calculations. Hopefully, the numbers converge in the successive steps and your idea of how the airplane and its components look like get more and more concrete.
Note that aspect ratio is not a design parameter at this stage, but merely the result of the chosen wingspan and area.

Has the $\left( 1.2 - \frac{m_{\textrm{Fuel}}}{m} \right)$-term been added lately? In my edition of Part V - Component Weight Estimation that term is not there there

@Robe Hey, cool, someone who checks my equations! I converted the numbers to SI units and added the fuel part from the German LTH. For a 20% fuel fraction the results are the same. Note that I also replaced n$_{ult}$ with 1.5·n$_z$.

Ah, good to know! Last year I did some mass estimation of FRP wings with a high aspect ratio. I ended up calculating my own fudge factors, but of the three formulas for GA planes provided in Roskam's book, this one had the largest error. Reading your answer, I though that it had perhaps been updated.

AR isn’t really something that falls out. It’s something that’s set/chosen by the designer.  Once you know how much lift you need to generate, and given the design lift coefficient you can compute the wing area required.  At that point you get to choose whether you want long skinny wings or short stubby wings.  Other design considerations may come into play here. Is wingspan limited by hangar constraints? Or airport terminal constraints. What kind of roll rates do I want to achieve? How much fuel will I need to store in the wings? For a given length how much will the wing weigh? You either decide what AR you want and go with  span and chord that gives you or you decide what span you want and go with the chord and AR that gives you.
What your book probably meant was that you could look at existing planes with similar specs and choose something similar.


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How to land using horizon?
 source
I recently heard pilots discussing a landing technique that some instructors are now teaching.
To assist new pilots in flaring, they instruct the pilot to adjust their attention toward the far end of the runway or horizon. There was some discussion that it won't work for a pilot short in height. As they look toward the end of the runway or horizon they are suppose to use some type of reference, such as "the horizon expanding".
What is this technique for landing and how is it suppose to help?
Update: According to the "Rod Machado" YouTube video provided below, it is the runway width that is the primary reference, not the horizon or end of the runway. The pilot is viewing the end of the runway for peripheral vision of the width.

This is the technique I was taught 10 years ago..

Seat heights are adjustable.   A 6 foot tall pilot and a 5'4" pilot should have their heads in the same place, and have the same external view by adjusting the seat position.

Many if not most aircraft do not have height adjustment for the seats... Being 6'3" I have the opposite problem. Of the 20-30 aircraft I have flown over the last 45 years, only a couple newer Cessna had height adjustment.

I really don't see how either the runway width or looking at the end of the runway could be a reliable guide to landing, since they're so variable.  Lengths go from a couple of thousand feet to a couple of miles, widths from not much wider than the wheel track to something I could probably land on crossways with a good headwind.  And then there's dry lakes and such...

@ jamesqf  I understand and do not use this technique myself. But a good explanation of using "angles" is provided by Wayne Conrad below. I do know from computer graphics programming and the obvious real video provided by Rod Machado that the closer you get to an object (in this case runway width) it will increase in size exponentially. I do see merit in students using this as a start, but I think most will eventually "get the instinctive feel for it" and be less dependent on it. Who knows? I am going to give it a try next spring when I fly again.

It depends on the student.  That guidance is used to teach students the art of flying the roundout in order to better teach the how to evaluate their height above the surface.  Care must be taken there as excess focus at the far end of the runway can result in a high round out resulting in a very firm touchdown or worse.  The actual technique of evaluating your height above the runway just comes with experience and several hundred landings and can be altered by different visual perceptions like runway width, grade, etc.
I personally find that it is useful to focus about 200 ft down the unwary in front of you while evaluating your height based on this as well as the end of the runway and the sides using your peripheral vision.  It can help to then shift focus to the end of the runway during the roundout as most of your field of vision will be blocked during the roundout, particularly in 3-point landings in tailwheel airplanes.  Again sight picture depends on the aircraft, runway and several other factors.
Longtime CFI Rod Machado developed a really good method for new pilots to evaluate flare height based upon runway geometry that works very well for light aircraft.  Using this method can be a big help during your formative training, as you develop a good sight picture which can aid you in future landings in other types of airplanes.

Thanks, I didn't know Rod had his videos available on the internet and I will download all of them. I tried this on my flight sim and it really helps because the sim does not provide depth perception. I have always "flattened" the air plane flight out at 5ft and let it settle but that is hard to do at night. After 2500hrs of flying, I am still learning "student" things! (Students don't appreciate how much things have improved since my license in 1975 :(

Highly recommended for flight training, dude.  He publishes really good stuff.

He also answered my first impression (made me think this had no merit) that this doesn't work for larger aircraft because of the height of the cockpit above ground.

I think the GIST here is:
Look far ahead. Don't look at whatever is directly in front of you.
It's the same as driving. If one looks at the piece of concrete just meters ahead, then the barricade will "pop out in front suddenly" or the bumper of the car is "in your face out of nowhere".
Our peripheral vision will guide us to wherever we want. It is natural and in-born, biologically. You just need to know where to look.

This is how I was taught to land and perhaps the broader way to put it is,

Look out at the end of the runway or horizon and keep that sight picture in your head when landing

As noted in the comments seats are often adjustable or you can use a thin cushion to add some height if you need it. But even at that everyone will have a slightly different sight picture when landing and its important to keep that consistent. A point like the far threshold or horizon (if your airport is in a generally level area) provides a nice reference for you to keep your flair pitch consistent and repeatable.

Here's how my flight instructor explained it to me.
Human depth perception, beyond a very short distance, is very poor.  If you are looking at the runway directly ahead of your plane, it is difficult to judge your height, or changes in your height, with enough precision to make a good landing.
However, your ability to judge angles is very sensitive.  If you are looking down the runway, then you can see both the horizon and the runway edges.
 The runway edges and the horizon have angular relationships that vary depending upon your height.  This angle, and changes to it, are easy to see.
Therefore, by focusing down the runway, you are exchanging a poor mechanism for judging height--depth perception--for a better mechanism--angle perception.

In addition to Dave's correct answer about holding a consistent pitch attitude through touchdown, it also helps new pilots to notice and eliminate drift.  Once you have a few thousand hours you don't even realize you're doing it, but for a beginning pilot it's hard to make the correct inputs of aileron to correct sideslip and rudder to align the longitudinal axis of the airplane with the centerline of the runway simultaneously (heck, it's even hard to type!).
By looking out at the far end of the runway a beginning pilot gets a better idea of how much rudder and aileron input is required.


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On Air France 447, what would have been the lowest altitude to initiate recovery after the stall developed?
What could the lowest altitude of Air France 447 have been to recover the (stalled) flight, where the co-pilot informed the captain that he was pulling up the whole time?
The co-pilot didn't inform the captain until 2000 feet, which obviously was too low.

GXL888T stalled at 2,910 feet at level flight; and despite the captain actively trying to recover, it didn't. AAF447 was falling with high sink rate, so that suggests the minimum recovery altitude to be much higher.

For the stalled flight to recover, the nose needs to be pointed in the airstream, and then the aircraft pulled up with load factor below the ultimate load. From the accident report:

The recordings stopped at 2 h 14 min 28. The last recorded values were a vertical speed of -10,912 ft/min, a ground speed of 107 kt, pitch attitude of 16.2 degrees nose-up

Initial situation.
From earlier on in the report we can conclude that these values were typical for the complete stall from 36,000 ft to impact, except for the pitch attitude which was around zero for most of the stall. The flight state parameters were therefore:

Vertical speed was 10,000 feet/min = 51 m/s
Horizontal speed was 100 kts = 51 m/s, which constructs an airspeed of 72 m/s.
The engines were at TO thrust, creating a nose-up moment, compensated by the nose-down moment of the airstream hitting the horizontal stabiliser.
The automatic trim is retained in alternate law, so the stabiliser is trimmed full LE down = 14º.
Elevator is fully up = 30º on top of the trim - it is almost aligned with the stalled airstream.

Nose down manoeuvre.

At 72 m/s the aircraft is at about TO speed, and nose rotation speed should be comparable to TO rotation speed - a bit slower because one side of the elevator is stalled. It takes perhaps 5 - 10 seconds to rotate the nose down 45º (an estimated assumption from my part).
When at 45º nose down, gravity accelerates @ 0.7g and the engines at perhaps 0.25g. At this acceleration airspeed builds up with around 1 g = 9.8 m/s every second. If we take averages for nose down manoeuvre time and acceleration, airspeed builds up with 4.9 m/s for 7.5 seconds = 37 m/s. So when the nose is aligned the airspeed is about 110 m/s, but still accelerating fast.
During the 5 - 10 seconds nose down manoeuvre, the aircraft loses about 1,000 - 2,000 ft in altitude

Speed increase
Before the aircraft can be pulled from the dive, the true airspeed needs to be brought to the manoeuvre speed $V_a$. From the A330 FCOM:

The load alleviation is only available when :

The aircraft speed is above 250 knots.
The FLAPS lever is in the 0 position.
In normal or alternate law flight mode.

Above 250 knots the load alleviation system is active, in order to keep the maximum load factor at 2.5g. The manoeuvre speed (or cornering speed in military speak) seems to be set at 250 knots = 128 m/s. At 1g acceleration, it takes 2 seconds to increase speed from 110 to 128 m/s, during which the aircraft loses altitude of 500 ft:
$$\Delta h = sin(45) \cdot (V_0 \cdot t + \frac{1}{2} \cdot a \cdot t^2)$$
Pull-up manoeuvre.
Then during the pull-up manoeuvre, the load factor must stay under the limit load = 2.5g. 1g is taken up by gravity, and there is 1.5g available for the pull-up manoeuvre.
$$ m \cdot \Delta n \cdot g = m \cdot \frac {V^2}{R} \Rightarrow R = \frac {V^2}{\Delta n \cdot g}$$
With the values established above, we get R = 10,000 / (1.5 * 9.81) = 1,100 m = 3,300 ft. But that is at constant airspeed of 128 m/s, in reality the airspeed will still pick up at the beginning of the manoeuvre and the radius will be higher - let's say 4,000 ft. The velocity vector of the plane was 45º pointing downwards, so half the radius is used. This is from the moment the AoA is close to zero, which it needs to be in order to start the pull-up, which increases AoA again.
My estimate for the altitude required for a successful pull-up from the stalled situation, is therefore 2,000 + 500 + 2,000 = 4,500 ft if they end up skimming the tops of the waves. If they know exactly what to do, time things perfectly, and manage airspeed perfectly. The aircraft is protected against pulling too many g's, so once the nose is aligned the flight crew can pull the stick fully aft and let the aircraft manage the minimum pull-up radius. If for instance the pull-up is initiated 1 second later than the 2 secs estimated above, the airspeed increases to 138 m/s and the pull-up radius becomes 1,300 m = 4,000 ft, plus the aircraft loses an additional 300 ft during the 1 extra second diving down under full power - this 1 second delay in initiating the pull-up requires another 1,000 ft.
Recovering from a fully developed stall is now trained in Level D simulators. Picture below is from a company that makes the flight model and instructor station extension for any flight simulator to train the manoeuvre. Disclosure statement: I have done business with them in the past.

This is such a good answer, I will wait a day before accepting, but can't imagine a more thorough one. Thanks. I do wonder why this wasn't trained until recently though, it seems like a no-brainer, but I guess hindsight is 20-20.

Yes indeed, hindsight. But a good thing that the aviation industry learns from previous accidents and trains for preventing them in the future.

Don’t you need time, with the AoA near zero, for airspeed to build up to something that will sustain your 3G +/- pull? Looks like you modeled pushing until zero AoA & then immediately pulling back again. Did I miss something else? Any estimate of airspeed attained during that push? Without sufficient airspeed, that 3G pull will put the aircraft right into a secondary stall.

@Koyovis Training could only begin after reliable post-stall aerodynamic models were developed which didn’t start for commercial airliners until a few years ago. Before, any post-stall simulator handling did incur risk of negative training due inaccurate post-stall aircraft handling behaviour.

The 30 seconds seems a bit...  arbitrary.

The report of GXL888T suggests that 1,000 ft would not be enough to pull up the aircraft. In that incident, the AoA recovered at 1,600 ft with nose down 54 degrees. At that moment the pilot input full aft stick and thrust idle, the aircraft impacted water 6 seconds later. It is also unlikely that the aircraft is capable of pulling and sustaining max G in that condition. Therefore I believe the 1,000 ft figure to be highly underestimated.

@kevin -- that brings up a great point... aft stick + idle thrust won't get you anything like 3 G's unless you have LOTS of airspeed already & are bleeding it off. So at some point, the thrust has to come up -- way up. Which increases the speed vector in the direction -- down -- that the nose is pointing. So rate of descent goes way up as well until pitch attitude nears level flight. The more guys examine this, the more I agree with your conclusion: highly underestimated.

@RalphJ The engines were at full throttle all the way down.

@cloud They trained stall prevention.  The test program for that family of aircraft did not include full stall and data collection that would allow for a good enough simulator emulation of a stall.  Practicing stalls in the aircraft is nowadays deemed to be too expensive for airliner class planes.  Training stall prevention, if effective (in theory)  should render the problem moot ... except when it doesn't.

@CptReynolds Yes the flight simulator data was only valid within a limited region of AoA and sideslip. Outside this envelope, extrapolated data was used but that is totally unrealistic and invalid, as AA587 demonstrated. Actual data from crashes was used in developing full stall aerodynamic models, plus dynamic windtunnel data: the windtunnel model was measured while being rotated. Fully developed stall behaviour turns out to be determined by the aircraft config: low/high wing, T-tail, engines underwing etc.

+1 Nice calculations. When asked the same question the Airbus engineers didn't even want to hazard a guess with the data they had

@Lnafziger I've attempted to quantify the time required for the nose alignment a bit better.

I have not yet worked through your calculation in detail, but on first looks I am not sure whether your speed is indicated airspeed (for minimum flight speed purposes it should be) or true airspeed (for geometric calculations you need that). In any case, at high altitude the two differ a lot. Maybe you would like to add a note on that in your post?

1g stalling speed from the AF447 report on the day in ambient conditions was around .65 Ma, which for 36000ft and ISA+15 is around 212kts IAS or 386kts TAS.

@CptReynolds It's true airspeed for the manoeuvres, I've added to the answer.

@Cloud: about the "no-brainer". Every pilot is trained from day one to fear and avoid stalls. The question in the case of AF447 is not about how the pilot dealt with the stall, but why he decided to not believe any of his instruments (they were all working fine) and keep full stick aft while falling from the sky, and not recognizing that the aircraft was in alternate law; together with poor CRM that allowed this to continue without the other two noticing.

You probably said this somewhere and I missed it, but where did you get the 100-kt horizontal speed from?

@Koyovis: "The engines were at full throttle all the way down" - Full throttle, but were they at full thrust?  At the extreme AoAs encountered during AF447's stalled descent, I'd expect there to be a considerable degree of engine-inlet-airflow-disruption-induced power loss, which the engines would take a nonzero amount of time to fully spool back up from even after lowering AoA to a sane value.

One more note: once you've pushed over to θ=-45º, won't the velocity vector have steepened past -45º (because gravity) by the time you initiate the pull-up maneuver?

Final (I think) question for now: "The velocity vector of the plane was 45º pointing downwards, so half the radius is used."  Assuming that the plane's velocity vector is actually -45º (see previous comment), won't the pullup to level flight take 1-cos(45º)=0.293 of the radius, not half of it?

@Vikki The 100 kts is a general representation, from the horizontal speed at crash and looking back at the general data before that.

@Vikki thrust & throttle - yes could be, the inlet flow could be partially stalled. Velocity vector pointing down: good point, less lift from a lower stalled wing flow. And yes for the cosine comment. Working flat out on a large project now, not sure if I have the time to update the answer (of 5 years ago )

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Why does the U-2 Dragon Lady not have swept wings?
Swept wings increase the critical mach number for aircraft with otherwise identical wings (Source 1). The U-2 Dragon Lady is limited in altitude by its critical mach number, as it needs more speed in order to go higher (Source 2), but going faster would put it into its "coffin corner" (Source 3). However, looking at the following photo of a modern U-2 from Lockheed's website, the wings seem to have little to no significant sweep. The top speed (presumably what is used during the highest-altitude cruising) is only Mach 0.67 (Source 4), while modern aircraft like the Boeing 737 (which do have swept wings) often cruise at speeds between Mach 0.75 and 0.8 (Source 5).
Why does the U-2 not have swept wings?

I know that wing sweep was first developed in Germany during WWII; had the technology not taken hold in the USA by the time the U2 was developed in the 1950s? Or is there a deeper technical reason I'm missing?

Flying fast isn't exactly what they want to do. They want to fly high to avoid being intercepted, but they don't really want to fly fast because they need to loiter around a target (or targets) to gather intelligence.

Perhaps for the same reason sailplanes seldom have swept wings?  The U2 is pretty close to a jet-powered sailplane...

I believe, there was simply no desire at all for this plane to go fast.

@RonBeyer, It is true that the re is a benefit to flying high to avoid anti-aircraft measures, however the benefit of sensor coverage is significant. The speed was determined to be limited to subsonic for several reasons, which included complexity and acoustic stealth. But the notion that a U-2 loiters around a target is perhaps a mis-understanding. I have personally reviewed hundreds, if not thousands of U-2 missions, and they are planned for linear flight line. Loitering increases risk, and does not substantially augment the intelligence value of the flight.

Sweeping a wing costs weight, for several reasons. Therefore, if the goal is to fly high and achieve long range, an unswept wing is the better choice. Only when transport performance needs to be maximized will a moderately swept wing look better.
Why is a swept wing heavier?

Sweep reduces aerodynamic efficiency, so a larger wing is needed to create the same amount of lift.
Sweep introduces torsion when a straight wing will mostly see bending only. Adding torsion strength needs more structure.
The stall characteristics of a swept, high aspect ratio wing are extremely unpleasant and require a healthy margin from the stall angle of attack, when flying high requires to fly just at the edge of stall.

In the end, Kelly Johnson decided to cut weight and drag as much as possible but not to pursue maximum speed to achieve the greatest flight altitude. Another reason was that the U-2 begun as a modification of the XF-104 with extremely stretched wings. Adding sweep would had resulted in too many modifications to the U-2 fuselage, which was initially built in the same tooling than that of the XF-104.

Basically, because of its mission requirements. It wasn't intended to fly fast, and because it didn't need to go particularly fast, there was no need to sweep the wing. Instead, it was supposed to provide a massive amount of data (by 20th century standards) along its flight path, which necessitated long flight times: thus the long, straight wings providing a lot of lift at low airspeeds were ideal for its mission.

A swept wing would most likely adversely affect the handling of a U2 during a typical mission profile, and would adversely affect the structure required (weight) of the aircraft.
A swept wing adversely impacts the stall characteristics and stability when flying near a stall.
It should be noted that while a swept wing helps manage effects due to the compressibility of air near mach speed. The primary effect is a swept wing delays the near mach shock wave development. This results in reduced drag near transonic speeds.  The U-2 is designed to operate just below transonic speeds, so in theory, there could be a benefit of a swept wing on the U-2.
The benefits of a swept wing comes at a price.  In addition to requiring a beefier structure to handle the torsional load stresses of a swept wing, the stall characteristics of the wing are different.
The resultant stall characteristics can become more pronounced and more difficult to recover from. In the case of the U-2, which at higher altitudes can fly with as little as a 2.5 to 7.5 knot gap between stall and MMO, the pilot must be sensitive to the pre-stall buffet.
A swept wing would make the sensing of that buffet potentially a little more difficult, but more importantly, it would make stall recovery much more difficult.  In many aircraft that would be manageable, however, in the case of the U-2 stalls are not docile, and have been known to cause tail structural failures and separation.
The risks of a swept wing on the U-2 outweigh any benefit of reduced aerodynamic drag from a swept wing, given that there was a decision to have the U-2 be a subsonic aircraft. The additional weight of the structure needed for the swept wing could also be construed as to further close the stall to MMO gap.
Therefore, not implementing a swept wing on the U-2 helps control the stall characteristics, and should the onset of a stall occur, it is easier and with less risk, to recover from a stall.

The limiting factor for U2's speed is not the wing, but the engine.
According to Unlimited Horizons (also  U2 Developments ), engine performance is only 6% of sea level at 70000 ft due to oxygen starvation. Before engine is improved to the point that the same airframe can reach M0.8, there's no need to sweep the wing.

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What are the differences between the SR 71 variants?
I read that all SR 71 blackbirds were different, what was different with each one and is one regarded as superior to the rest? Why?

I'm curious where you read that.

Like every military aircraft built over the years, the aircraft evolve.  So to address the aircraft's history and all variants, let's refer to it as the Blackbird.
The Blackbird started out as the A-12 procured for the CIA. Fifteen of these were contracted to be built with one being a 2-seat trainer. Two of the 15 were contractually changed to M-21 aircraft prior to delivery.  The M-21 was specifically designed to launch the D-21 drones. The SR-71 at the Intrepid museum (60-6925) in New York was built as an A-12.  One of the M-21's is at the Museum of Flight in Seattle with a D-21B drone.
Three high-speed interceptor prototypes were built and designated as the YF-12A and were capable of carrying 3 air-to-air missiles.  Two crashed and the third is at the National Museum of the AF.
The interceptor was cancelled and the RS-71 program, renamed SR-71 after LBJ's misstatement at the press conference, went forward for the AF.  Twenty-nine SR-71A's were built along with 2 SR-71B trainers.  When one of the trainers was lost in 1968, the surviving aft part of one of the crashed YF-12A's was mated to the front part the SR-71 static test aircraft to create the sole SR-71C.
There's a huge Blackbird community online.  Detail specs and history are available at blackbirds.net and at SR-71 Blackbirds.
The question of which is superior is an opinion, but one can assume that the SR-71's were "better" than the A-12's as they came later building on what was learned from both the A-12 and YF-12A.

You may be referring to the fact that pilots felt that all Blackbirds seemed to have a distinct personality e.g. 962 was a hangar queen - a real lemon - whereas 972 never let you down.  Blackbirds were virtually a hand built airplane but as to whether each one was different may have just been a matter of opinion.

There are a few variants of the SR-71 but there is only 1 that was ever used in an active military role.
The YF-12 was the SR-71's initial test airframe. Of the three airframes build tests were carried out by the Air Force and NASA. Notice the nose is slightly different than the eventual production version.

(source)
A twin cockpit instructional SR-71 (SR-71B Serial Number 17956) airframe was built to train the pilots.

(source)
As far as I know the SR-71 only had one airframe variant, the SR-71...

(source)
The airframe was however capable of carrying a wide variety of payloads as dictated by the mission. You can find a nice overview of them here.

The YF-12 was the interceptor variant built for the USAF, and it was based on the A-12 - the YF-12 was not a development model for the SR-71, it was a completely separate development.  There were three SR-71 variants, the A and B (trainer) and the C was built from a crashed YF-12 and a static SR-71 test rig.

If you measure them in stealth, max altitude or speed. A-12 comes out on top. (not that stealth ended up mattering as Russians started to hook up computers to their radars and would then be able to spot the A-12s, they were never flown over Russia for that reason.)
If you measure them in surveillance equipment or range, SR-71 would come out on top.
If you measure in armament YF-12 would come out on top.
(image from cia.gov article OXCART vs Blackbird: Do You Know the Difference?)


text:
How did the two F-35 planes get from Fort Worth to Williamtown?
I am curious as to how the the F-35s that recently arrived in Williamtown, Australia did arrive there.
Were they transported at any point, or were they flown the entire way?
How many stops did they make?
Did Australian pilots land them in Australia? If so did Australian pilots take off in them from Fort Worth?

The two aircraft had been delivered to the RAAF at Luke in September and October and delivered to Williamtown via Hickam Air Force Base in Hawaii and RAAF Base Amberley, south-east of Brisbane.

From Defense News.

So did they have to do two mid air refuelling sessions from Fort Worth to Hickam Air Force base?

There is an art and science to scheduling refueling on long overwater legs.  The tanker stays with you and the goal is to keep you topped off so that you can make the nearest divert field.  They will never let you get too low.  Rather than one or two long drinks you will get multiple sips to stay topped off.  Hard to say exactly how many though.

Hmm interesting, I wonder if the fuel was complimentary or the US invoiced them.

My guess is US already include fuel in the bill.

When Australia received two F-35's in February of last year they were flown by Royal Australian Air Force pilots according to This article. So I would imagine they did the same for the most recent delivery. Pilots were flown to Fort Worth and they took command of the fighters and flew them back to Australia.
According to 9news "The two F35s will spend several days making a relatively slow trip to Australia touching down in Hawaii and Guam as well as air to air refueling."


text:
Why do some jet engines have fewer fan blades than others?
I have found out upon researching jet engine fan blades that they can vary from 16 to 34. Why do they vary, I mean, does decreasing or increasing the number of fan blades have any performance impact? Does this have any scientific/technical reason?

Why does this surprise you?  Airplanes and engines come in a vast variety of sizes, and have been designed by many different people over decades.  Just like piston engines come in different sizes, configurations, number of pistons, camshaft profiles, etc.  I’d be far more surprised if I discovered that every jet engine had the exact same number.

This does not surprise me. I want to know what is the reason behind this difference

Consider rephrasing to "what determines the number of fan blades on a jet engine?"

Well, one reason is that engines come in many different sizes.

One possible narrower question: GE90-115B has 22 fan blades, GEnx (a derivative of GE90) has 18, GE9X (a later derivative) has only 16.  These engines are all high bypass turbofans, on the same order of magnitude of thrust, same manufacturer, designed for large Boeing airframes.  You might ask why the trend of decreased fan blades within this engine family

Yes exactly GE90 22, GEnx-18, GE9X-16. Is there any scientific/technical reason for fewer fan blades

Re "Why do some jet engines have fewer fan blades than others?"-- because some jet engines have more fan blades than others.

This might not be a super satisfying answer as I'm not a fan expert, but since no one else has answered, let me tell you what I know.  There are many competing and often mutually exclusive constraints on the fan module design.  This include, but are not limited to:

Overall weight
Efficiency
Thrust generated
Stall margin
Aeromechanics (i.e. flutter or high cycle fatigue issues)
Ability to resist a bird strike
Ability to contain a fan blade out event
Manufacturing cost
Maintainability / Repair cost

(many existing answers on this site about most of this topics if you are not familiar with them).
The number of fan blades is one knob that the fan designer can play with in order to optimize all of the above.  For some of them more blades is better, and for others more blades is worse. But number of blades is not the only knob, there are many more.  Other variables include, but are not limited to:

RPM
Fan diameter
Nozzle exit area
Fan blade material
Stagger angle of the blade

In short, there is a complicated optimization problem, where you are trying to turn 5-10 knobs in order to find the best combination of 5-10 desired quantities.  In order to find the best configuration, Designers will use experimental wind tunnel measurements, 3D computational fluid dynamics, as well as experience on previous engines.


text:
How to actually draw a wing properly
I’ve been trying to draw a wing with data such as
Ctip, Croot, b, lambda (Taper ratio) and S
What I’m struggling with is how to actually start with this if we don’t know the angle created by the fuselage with the as trailing or leading edge shown in my picture
So when we first draw the Croot and a Ctip a distance away from each other, how far to the left or right  would I draw Ctip?

What is lambda in this particular case? Leading edge sweep, 25% chord sweep or what?

Is that diagram intended to represent a wing?

@MichaelHall Yes, that is how you graphically determine the mean aerodynamic chord. See this for an explanation.

@PeterKämpf, thank you.  Your diagram is MUCH better, and I see a wing now.  All I saw this morning was a messed up looking Roman numeral twelve...

If you have c_tip, c_root, b and S and assume a trapezoidal wing (straight taper), you have one too many values prescribed, i.e. you can omit one of the four. If lambda is indeed taper ratio, then it's the same as c_tip/c_root, again redundant. But then you don't know anything about sweep.
If Lambda is sweep angle, on the other hand, then you know everything you need. For subsonic aircraft, sweep angle usually denotes the sweep of the quarter-chord line, so you can start drawing the root chord along the fuselage, draw a perpendicular (to the fuselage axis -> spanwise) line from this point, and then draw a line at the sweep angle to this perpendicular one. This will be your quarter chord. At the semispan, draw a line parallel to the root chord with length c_tip, extending 1/4 c_tip forward from the quarter chord line you just drew, and 3/4 back.

$C_{tip}$ is at a distance of ½b away from $C_{root}$. Placement left or right in the drawing follows from the quarter chord sweep angle.


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Why do different manufacturers recommend different throttle settings at startup?
I was wondering why different engine manufactures recommend different throttle openings before engine start? On many Pipers 1/4 Inch is recommended while the Cessna 172N recommends 1/8 Inch and the Cessna 172S again recommends 1/4 Inch.
Why do we open the throttle before starting the engine? What difference does it make opening the throttle more or less, has it to do with the fact that it isn't good practice to have high engine power when the engine is still cold and oil pressure is low? And why should we open the throttle differently when the engine is flooded or we start the engine when it is still hot?

The actual distance of movement isn't important, you don't want too much as you say, but it's also hard to start with a closed throttle. Typically I have one hand on the throttle and advance, retard as needed during cranking.

It's just a ballpark value that someone decided on during testing, that provides enough air to the engine to let it start and run with a rich primed mixture, without causing too much initial RPM.  A quarter inch here, and eighth inch there, it's all good as long as it doesn't cause the engine to burst into life at high power, and nobody expects you to sit with a measuring stick to get it exactly right.
It's not a good idea to start an engine with a very high throttle setting, because high rpm before there is sufficient oil flow = bad in the long run.  Crank and rod bearings need to be fully pressurized before any significant load is put on them. Cam followers need to be well coated in oil before they spin at more than minimum speed (especially with Lycomings).
Most engine wear happens in the first 15 seconds of starting, when the oil flow is inadequate and bearings, piston rings, cam lobes and gears etc are running on only a residual oil film.  Once a plain bearing is fully pressurized with oil, there is no metal to metal contact and the life of the bearing, in theory, is infinite.  If you ran a 2000 hour TBO engine with just one start and let it go without ever shutting down and just kept it supplied with fuel and oil 24/7, it would run far far beyond its normal TBO (although it might need cylinders at some point as valves wear out).
My personal practice is the use the absolute minimum throttle opening that will let the engine start, and let the engine tick over at minimum speed until I see normal oil pressure, before I go to a warm-up RPM. When an engine is warm, that usually allows starting with the throttle completely closed. Oil gets to where it needs to pretty fast; it's just that initial 10-15 seconds that's critical.

The record is 1559 hours in one flight, which only ended because they had lost so much power that they could no longer climb out after refueling. The engine started with 450 hours, so it barely made TBO despite continuous operation.

I made an edit to add that things like valve seats will tend to wear regardless so it may not apply to the entire engine.  That being said, Lyc O-320s regularly go to 3000 hrs plus, even the cylinders.  The engine in question may have been abused or mis-operated at some point - the exhaust valves are the biggest Achilles heel, and on Lycomings, cam lobes and followers if the engine has sat outside for extended periods.

@StephenS Would you have the source of this ? I would love to read about it.

@StandaaReinstateMonica AOPA article on it

As Ron notes in the comments the actual distance is not as important as the fact the throttle is "a bit open".
First off when comparing Pipers and Cessnas remember that they have different throttle actuators. Cessna uses the push-pull/vernier style throttle and Piper uses the throw style both of which are adjusted differently so 1/4 inch on either is not really the same thing. There is also room on the back side to adjust it, Ive flown Pipers where the first inch or so of throttle did nothing, or everything....
Opening the throttle ~1/4 inch is generally an ISO Atmosphere idea and warmer/colder days may call for different operations. The reality is if you poke around you will find advice in all directions. Pilot Workshops notes a 0-Throttle start this informative article notes basically "do what works" and some hot start procedures call for full throttle.
Ultimately most GA, carbureted aircraft are set up to idle nicely with the throttle against the stop when the engine is warm so when starting, with cylinders full of primed fuel, you may need a bit more air.


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What is a compressor cascade?
Is a compressor cascade just the representation of the blade ring of an actual turbomachine? What is the difference between linear cascade and annular cascade? A diagram or a reference would be great.

In mechanics, a cascade is the same equipment put in series, the output of one being the input of the following one. You may precise why you think this is different for compressors so that answer can focus on your interrogation instead of the general mechanical concept.

A compressor cascade usually refers to a non-rotating test setup to assess blade performance.


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Why are the Constellation's nosewheels canted inwards?
Why are the nosewheels on the Constellation canted in a few degrees? Is it to ensure both tyres have ground contact during tight ground turns?
I've observed that on the A320 during tight turns that one wheel comes completely off the ground. The A320 nose leg also sits at an angle; would it benefit (if needed) from angled nosewheels?

http://www.ipmseaglesquadron.org/ES-Forums/index.php/topic,1639.0.html

Are you asking about the cant of the wheels themselves (I would call it out, not in, though—the tops are further apart), or of the strut?

@JohnK gives an excellent explanation for castor - the horizontal offset between the steering axis and the tire contact point.
But maybe the question is about camber. That is the angle between the vertical axis of the tire and the vertical direction relative to the ground. The camber angle on the Connie's nose wheels is called positive camber, and an extreme case can be found on the tractors of that time. Curiously, it is a distinct characteristic only of US tractors like the John Deere (left) and Farmall (right) tractors below (picture source).

The reason is easier steering in rough terrain. As a tractor discussion board explains:

I have read that positive camber is intended to align the kingpin axis with the center of the tire contact patch. Supposedly this reduces the force required to steer the axle. If that theory is true, then changing the height of the tires would change the intersection point of the camber angle and the kingpin angle; it's improbable that swapping between, say, the correct R4 tires and the correct R1 tires for your tractor would make significant differences.
However, I am not convinced by the theory that the positive camber is there for steering ease. Here is why: I think it would be easier to roll a tire around in a circle than it would be to simply twist it, especially if one considers the tire will be pushing dirt out of the way as it twists.
I think the reason for the positive camber is to attenuate the camber thrust that can occur when the tires encounter uneven terrain. This would tend to help the tractor track straighter, and require less steering input to maintain a straight course. That characteristic, along with the caster geometry, should make the tractor track straight naturally, rather than tending to turn more when either a steering input is made or some uneven terrain is encountered.

The author of this post has his own theory; however, the point made in his first sentence actually has merit: Since steering in automobiles and European tractors is made easier when the steering axis runs through the contact point of the tire with the ground, positive camber is an attempt to reduce the steering roll radius of the twin tires which indeed should make steering easier.

Steering axis inclination reduces the roll radius in cars (picture source); since the sideways axis inclination could not be used in nose gears or tractors, both resorted to tire camber.

Yep I was convinced he was talking about the angle of the main strut. My bad.  Thanks Peter.

@JohnK: Me too, at first. Only Jan's comment made me read the question again.

The angle of the main strut remains important to the design, as the wheel must roll slightly in the turn. "Castoring" seems to be actually be an unstable condition, there for a castoring wheel should "trail". However, in the rake, the roll factor keeps it from turning when gyroscopic forces are present, but it "castors" unstably at low speeds, making turning easier. This is why John's initial work was appreciated.  The cant probably made the farmers wheel shake a lot less as uneven terrain was crossed.  Plus the inward leaning angle definitely schmooshed the soft ground aside.

One problem that strikes me right away with the Connie's cambered wheels is during a high speed directional input the roll-under load on the "up-turn" tire must be massive.

@JohnK Camber is an important tool in tuning a race car to go around corners. While an F1 car is considerably lighter than the Connie, cornering at 150MPH pulling 5G puts a fair bit of stress through the system. Tires don't roll off their beads.

It's basically for the same reason motorcycle forks are canted or raked.
For a nose gear, you have two options to give the wheel assembly a natural self-aligning, or castering, tendency.
You can make it work like a shopping cart caster like the sketch on the right, with a vertical strut and the wheel on an offset dogleg fork so that the wheel contact point B is aft of the steering or pivoting axis point A.  A lot of airliners do it this way and it has the advantage of a flat contact pattern all the way around, as well as the  ability to caster 360 degrees when the scissors is disconnected.  But it's heavier.
If you want to make the strut straight without the dogleg, which is a bit lighter, you  make it like the sketch on the left, and angle the pivot axis of the strut so the pivot axis intersects the ground forward of the wheel's axis (called the caster or rake angle - the distance between A and B is called "trail"). Like a motorcycle fork, or a car's front suspension.  This gives a natural castering tendency, which is why you can take your hands off the handlebars on a motorcycle and it doesn't veer off.
Raking the strut is almost the universal configuration on single tire nosewheel light planes, but doesn't work that well for turning with double tire wheels because one lifts off the ground when it turns, putting a lot of stress on the axle and wearing the tires unevenly, but this is significant only when making fairly sharp turns so you can get away with it because things are moving slowly. On the other hand, double tires also enhance the rolling stability because both tires want to stay planted by gravity and the only way is when straight - easy to imagine if you mentally increase the rake to 90 degrees - so you need less steering rake than you might otherwise need with a single tire.

I don't see how this answers the question. Motorcycle has only one front wheel that is not canted. However, Constellation has two wheels on the nose gear, and they don't have common, level, axle, but are tilted outward.

Motorcycle forks have an angle to allow the rider to control the leaning of the bike by altering the aligning of the axis of the two gyroscopes (aka wheels). It has nothing to do with self aligning or shopping carts wheels. And the reason a motorbike stays up is because of the gyroscopic inertia of the wheels.

@Jan Hudec The canting helps LOW SPEED steering.  Think of the 2 wheels as right and left halves of one wheel. But once the "wheel" spins up, gyroscopic physics takes over.

https://en.wikipedia.org/wiki/Caster_angle

@JanHudec  The FORK of the motorcycle is canted not the wheel.  Rake angle. The fact that there is a double tire is immaterial.  The Connie's nose strut has a rake angle for the same reason a motorcycle fork, or a bicycle fork, or the steering axis of a car front suspension is canted.

@DiegoSánchez the motorcycle's stability comes from the combination of gyro stability, rake angle that makes the wheel naturally want to trail, and constant and subtle corrective pressure application inputs on the handlebars by the rider.  If you tried to ride a motorcycle with a vertical fork so there is no trail, the gyro stability of the spinning wheel won't help you and you will find the machine uncontrollable.

@JohnK, please, read the question again, and look up a picture of a constellation. The point is that the wheels themselves are canted, not the strut/fork/rake.

@JanHudec  the question is referring to the canting or rake of the strut itself, which on the Connie is about 5 degrees.  ANY nosegear where the wheel axles are right on the strut axis will have some rake.  The distance between the strut center line and the wheel axis at the surface is called trail.

@JohnK, are you really sure? The part “the A320 nose leg also sits at an angle; would it benefit (if needed) from angled nosewheels” clearly indicates that “angles nosewheels” means something else than the strut.

I suspect J Frew's question is somewhat unclear.  When I look at the 320 strut, I see a short stubby version of the Connie's strut.  You can see the rake angle, which is essential with non-offset nosewheel.  On the other hand, if you look at a 777 nose gear, you can see the strut is vertical but there is a very subtle offset of the wheel axle of maybe 2 or 3 inches aft.  It gets the necessary steering trail that way.

He's talking about the camber of the individual tires, not the caster of the strut.

Yes, see me smacking my forehead?  Smacking now...

Have to give MikeY, Quiet Flyer, and John K credit for helping me get this one, along with some reading on chopper (motorcycle) rake design.
When a forward rake is "turned", it rolls and yaws, resulting in the tire being slightly on its side while it is turning.  This is why the Constellation tires are canted a few degrees.  The outside tire can lift completely off the ground as it rolls upwards when the wheel assembly is turned.
Why a raked fork?  We know a spinning disk has stability and will resist being turned out of its plane of rotation.  Since a raked fork rolls the wheel to its side, the assembly can only be turned with effort against the spinning wheel.  This stability would be very important to a long nose strut such as the one found on the Constellation.  Any wobbling would be disasterous.
Whether the A320 would benefit from angled wheels, if the rake or length of the strut is not as great as the Constellation, probably not as much.  But with the Constellation, it is expected that the inside tire will bear the turn.

You might be on the right track, but I don't really see the argument. The outer wheel lifts because the strut is canted, but I don't see how canting the wheels themselves (outward) changes it (and I do believe it has something to do with the stability—I just don't see what from your argument).

Some more reading revealed it makes farm tractors steer easier.  The rake is what gives the wheel assembly stability.

Raking the steering axis so it intersects the ground forward of the wheel axis makes the wheel want to trail.  The dimension of the two intersect points on the ground is called "trail" for that reason.  The rake angle itself is actually destabilizing statically, making the wheel want to flop over.  When rolling, the wheel overcomes this and want to trail, like a caster, and gyro forces provide additional stability.

And the pitch to the ground of the wheel is called "camber", oh my goodness!  But this is actually a very ingenious way to deal with the long strut of the Constellation, it does not want to turn at high speeds (just after touch down) but can be turned at low speeds.  The cant of the wheels helps by narrowing the ground contact, reducing "handle bar" torque (manual steering tractors love this), and the "flop" is the tire rolling on it side as well as yawing.


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Why don't air-cooled radial engines use fuel to cool the oil?
Having read up on WWII air-cooled radials, one thing that struck me is how frequent is the mentioning of external oil cooler overheating, or the need for armored boxes around it, and some even venture to say the so-called air-cooled radials are in reality oil-cooled radials.
This got me thinking. Since the oil in turbofans is cooled by fuel flow, why isn't this the case for radials? Is this because the fuel flow is insufficient, or the oil too thick, or some other reasons?

piston engine generates much more heat to oil while doen't burn nearly as much fuel.

The Fuel/Oil heat exchanger serves two purposes. It helps to dissipate some of the heat from the oil and warm the fuel up for combustion

The fuel-cooled oil cooler acts as a fuel oil heat exchanger in that
  the fuel cools the hot oil and the oil heats the fuel for combustion.
  Fuel flowing to the engine must pass through the heat exchanger;
  however, there is a thermostatic valve that controls the oil flow, and
  the oil may bypass the cooler if no cooling is needed.

In piston radials you generally dont need this fuel pre-heat. Since Avgas is vaporous at a fairly low temperature you dont really need to pre heat it. On top of that preheating it may vaporize it in the lines which is also a fairly big issue in and of itself.
There are always concerns of cross leaking, and this applies in both cases. A leaky exchanger can add fuel to your oil or oil to your fuel. A potential issue in either direction.

But the oil in the turbo engines is cooled by nothing other than the fuel flow. Perhaps the oil only cools the bearings and accessory gearboxes in the turbo, whereas it picks up heat all over the interior of the radial, and that results in a more significant fraction of heat going into the oil provided two engines of similar hp, e.g. twin wasp vs pt6?

Isn't the fuel-oil heat exchanger even more for heating the fuel in the tank? Jet-A freezes below -40°C and Jet-A1 below -47°C and the average temperature at tropopause is -54°C… Gasoline has lower freezing point and piston aircraft don't have as high service ceiling, so it is not a concern for them.